1. Technical Field
This invention relates generally to acoustic actuators and, more particularly, to flat panel loudspeaker systems.
2. Description of the Related Art
Most acoustic actuators (xe2x80x9cloudspeakersxe2x80x9d or simply xe2x80x9cspeakersxe2x80x9d) are relatively heavy. Further, the they act as point sources for producing sound. Many applications, such as virtual reality, entertainment, and active noise and vibration control, would benefit from loudspeakers that are extremely lightweight, compact and low-profile (i.e. flat), yet capable of high acoustic power emission with a high degree of spatial resolution.
Most existing loudspeakers use cones that are driven by electromagnetic actuators. Such devices have heavy permanent magnets and copper coils. To achieve high spatial resolution would require many such actuators. The weight of many such actuators would be quite high, limiting their use in automotive or aerospace applications. Further these loudspeakers are not low-profile. Low-profile actuators that are based on diaphragms driven by piezoelectric ceramics or polymers exist but do not have high acoustic power output capabilities because the motion of the diaphragm is small for piezoelectric devices. The control of airflow over a surface is another area that requires lightweight, low-profile and large displacement actuators with good spatial resolution.
Existing electrostatic loudspeakers are lightweight and low-profile. However, they have several disadvantages for many applications. Electrostatic speakers use air as the dielectric medium, with a single large continuous flat surface which radiates the sound as it is electrostatically attracted to one or two plates at different voltages. These speakers tend to be costly since it is necessary to carefully construct the speaker so that the large-area moving plate does not contact the stationary plate(s), and yet with a small enough plate spacing to permit using a reasonably-low driving voltage. Electrostatic speakers typically operate with a bias voltage of several thousand volts. Limitations on the driving voltage also limit the acoustic power output.
Acoustic actuators based on the electrostriction of polymers also exist. This type of actuator produces motion from the electrostriction of various polymer films, that is they produce sound primarily by the change in thickness of a polymer film (or stack of films) due to the electrostrictive effect. The displacement of the surface of this device is small compared to its thickness and so the acoustic power output is low.
This invention relates to an acoustic actuator or loudspeaker system. More specifically, the invention is a device that is capable of producing sound, vibrations, and changes in the shape and roughness of a surface in a fluid medium. Most commonly, it is anticipated that the device will be used as a loudspeaker in air. The invention is also well suited for use as an acoustic actuator for active noise and vibration control systems. The invention may also be used in non-acoustic applications, such as the control of airflow and turbulence on the surface of aircraft, ships, or other objects.
In one embodiment of the present invention, a loudspeaker (xe2x80x9cspeakerxe2x80x9d) is provided that is extremely lightweight, compact and low-profile (flat), yet capable of high acoustic power emission. The speaker is easy to manufacture and uses low cost materials. The speaker is flexible and, since it is essentially flat, it can be attached conformably to flat or curved surfaces as if it were an external skin or cover. It is also possible to make the loudspeaker largely transparent, allowing it, for example, to be placed over windows.
The above features imply that the speaker is well suited for applications such as noise cancellation and vibration control where a large radiating area and lightweight are important. Thus, the invention is an improvement over xe2x80x9ctraditionalxe2x80x9d speakers that employ electromagnetic or piezoelectric actuators. Those devices require roughly eight and five times, respectively, the actuator mass in order to produce the same power at a given frequency as an acoustic actuator of the present invention. Additionally, since the present invention is formed out of many small elements, individual elements, or groups of elements, can be driven individually for improved spatial resolution. The integration of sensory capabilities (such as a small microfabricated pressure sensor or accelerometer; or using capacitive measurement of the polymer itself) with each actuator element forms the basis of a xe2x80x9csmart skinxe2x80x9d that can automatically cancel noise or vibration.
Since the elements of the speaker are capable of large deflections, the device has non-acoustic applications as well. For example, an array of small elements may be used to influence the flow of air or other fluids over a surface.
The invention is fundamentally an electrostatic speaker; however, it has important differences from existing electrostatic devices that allow for greater power output, lower operating voltages and a simpler and more versatile design. Traditional electrostatic speakers use air as the dielectric medium, with a single xe2x80x9clargexe2x80x9d continuous flat surface which radiates the sound as it is electrostatically attracted to one or two plates or grids at different electrical potentials, while this invention uses an elastomeric dielectric. Furthermore, the present invention is composed of one or more discrete elements or xe2x80x9cbubblesxe2x80x9d that radiate the sound. These differences give the invention distinct advantages over traditional electrostatic speakers in that they permit greater acoustic energy output, lower driving voltages, greater shape versatility, and greater ease of manufacture.
The presence of the polymer dielectric between the electrodes eliminates the need to precisely control the electrode spacing. Dielectric films as thin as 1 micrometer have been demonstrated to operate at approximately 100 V. Electrostatic speakers typically operate with a bias voltage of several thousand volts. The division of the radiating surface into discrete elements eliminates the need to maintain flatness of the radiating surface, allowing the invention to conform to different surface shapes.
The polymer dielectric in the invention allows greater power output (per speaker surface area and weight) at a given voltage, since the electrostatic energy is multiplied by the dielectric constant of the polymer (typically between 2 and 10). In practice, the polymer dielectric will have a greater breakdown voltage than air, due partly to the fact that the polymer prevents the accumulation of particulates on the electrodes. Thus, the electric field generated by the applied voltage can be greater than air-gap devices, further increasing the power output capabilities of the invention (power output is proportional to the square of the electric field).
The invention may also be considered to operate based on the electrostriction of a polymer film. However, it differs from other electrostrictive devices that produce sound primarily by the changing the thickness of a polymer film (or stack of films) due to the electrostrictive effect. In contrast, our invention produces sound by using in-plane strains to induce out-of-plane deflection the film. The apparent stiffness and mass of a polymer film operating in this out-of-plane configuration can be orders of magnitude less than that for compression of the solid polymer as in other electrostrictive devices. The air driven by the film has low mass and stiffness. Thus, the invention is better coupled acoustically to the air resulting in greater acoustic output (per surface area and per weight) for a given electrical input.
The invention depends on a form of electrostriction of a polymer dielectric. However, the mechanism of actuation in the invention is believed to be different from the electrostrictive devices that rely on the change in thickness of the polymer to produce motion in that here the strain results principally from the external forces caused by the electrostatic attraction of the electrodes rather than just from internal intermolecular forces. This distinction gives the invention the advantage that the dielectric materials can be selected based on properties such as high dielectric strength, high volume resistivity, low modulus of elasticity, low hysteresis, and wide temperature operating range (which give advantages of high energy density, high electrical to mechanical energy conversion efficiency, large strains, high mechanical efficiency and good environmental resistance, respectively) rather than just the magnitude of the electrostrictive response for a given field. Dielectric materials with the aforementioned properties (e.g. silicone rubbers) have produced strains over 25%. The literature describing electrostrictive polymer actuators using rigid electrodes does not show any material with an electrostrictive response of this magnitude. Further, electrostrictive materials do not necessarily have a large response in the in-plane directions and, therefore, cannot effectively make use of the out-of-plane deflection mode of operation. Other devices known in the art also do not teach that compliant electrodes are important for operation of the devices. Compliant electrodes are important to the present invention, as they allow for the development of large strains.
The use of polymers with low moduli of elasticity also allows for high acoustic output per surface area and per weight at lower driving voltages than possible with other devices since the resulting motion at a given voltage is greater with the more compliant materials.
The individual elements that comprise the speaker in the invention can be extremely small or large. If small, the elements can be made with microfabrication techniques. Other speakers that function based upon the bending of a small microfabricated diaphragm exist. Such devices employ a silicon micromachined diaphragm. In such devices, the diaphragm may be driven with piezoelectrics or electrostatically. The cost of silicon and of piezoelectric materials greatly exceeds that of the polymers used in the invention and so the total surface area of these devices is limited. Additionally, the maximum energy and power output of the polymer speaker is greater than that of piezoelectric or electrostatic devices on a per weight or per surface area basis up to frequencies of several thousand Hertz. This frequency range is very important in sound production and in noise and vibration cancellation.
Loudspeakers made in conformance with the present invention can be attached conformably to flat or curved surfaces as if they were an external skin or cover. This configuration allows the loudspeaker to cover a larger area with improved spatial resolution of the sound. Other audio applications, such as consumer household or automotive audio speakers and loudspeakers for multimedia and virtual reality presentations can also benefit from improved spatial control of sound and low-profile speakers that could unobtrusively be located on walls, ceilings or other surfaces. Many consumer applications also require that the actuators be easy to manufacture and use low cost materials.
These and other advantages of the present invention will become apparent to those skilled in the art upon a reading of the following descriptions and a study of the drawings.